# Help finding propeller thrust.

1. Nov 5, 2012

### unethikal

Hello, I am working on a project regarding truss analysis on a plane and I need to find all the forces acting on the truss in order to produce static equilibrium (note that I have not yet taken fluid mechanics and I wanted a challenge).

From what I have found, propeller thrust is:

Thrust = Mass airflow X (slipstream velocity-aircraft velocity).

The aircraft is moving at a true airspeed of 120km/h. Do you guys have any idea how to calculate mass airflow and slipstream velocity with these variables? The propeller being used is this:

http://en.wikipedia.org/wiki/Bristol_Pegasus

Thank you.

2. Nov 6, 2012

### CWatters

The short answer is that thrust will be roughly equal to the aircrafts drag (some asumptions have to be made). Google the four forces of flight.

I don't think you can calculate thrust from the data you have. I suppose you could have a stab at calculating the thrust if you knew how much power was required to go at 120km/h and made some assumptions about prop efficiency.

3. Nov 6, 2012

### CWatters

PS You could look up data for the top speed of a Vickers Wellington powered by two of those engines. Knowing the power and that the prop might be say 80% efficient you could take a stab at the thrust needed to fly at that top speed.

4. Nov 6, 2012

### rcgldr

That's similar to the equation for rocket thrust. The term "exit velocity" is used to describe the velocity of the affected air at the moment its pressure returns to ambient. Nasa article:

propeller_analysis.htm

This thrust equation isn't very useful, since it's essentially just two ways of expressing thrust after you've already calculated what the thrust is.

What you want is a "thrust calculator" that uses propeller characteristics (blade shape, blade size, number of blades), rpm, and aircraft speed. You can try a web search for thrust calculator, but most of these will be for "static" thrust, where the aircraft is not moving. You can also try searcing for "blade element theory", which will provide more mathematical based articles. Here is one web article that goes through the math, but I don't know if it will provide the answer you're looking for, or how accurate it is (not sure if it takes into account the induced flow effect on a prop with more than 2 blades):

prop1.htm

You may be able to find other and more useful articles.

Last edited: Nov 6, 2012
5. Nov 6, 2012